Thin film deposition of lithium is typically done by evaporation. Lithium physical vapor deposition (“PVD”) targets for e.g., sputtering equipment are not commonly available; and lithium coatings formed by sputtering processes employ planar targets, not rotary targets (also referred to as tubular targets). Unlike planar targets that are pressed or forged, rotary targets are formed by casting, a method that allows control of grain size, including the uniformity of the grains. Providing a uniform grain size distribution, on the other hand, is difficult when one presses lithium to form a target. They also are limited to use in certain types of deposition equipment.
Rotary targets provide several benefits in comparison to planar targets, including longer life, higher target utilization, and improved ability to withstand reactive sputtering environments via the self-cleaning feature of rotary targets. In particular, planar targets have a stationary erosion pattern on the surface. Near the edges of the erosion pattern (race track), material is re-deposited, typically as an insulating oxide. This re-deposition encroaches on the erosion pattern and makes the erosion pattern narrower, i.e., the amount of material available for deposition decreases. A similar erosion pattern exists on a rotary target, but because the target is rotating, new material is moved into the erosion pattern. In other words, the redeposited area of a rotary target is moved into the erosion zone before it gets too thick and is “cleaned.”
Moreover, in the lithium battery industry, a strong demand exists to replace evaporation of lithium with sputtering. Evaporation produces a highly uniform deposited film. In order to achieve such films, however, substrates have to be arranged in a near parabolic configuration with respect to the evaporation source. Accordingly, unlike sputtering, evaporation is not well suited for the coating of webs and panels.